Display Device

ABSTRACT

A display device of the present invention has light-emitting devices making up a plurality of pixels placed in a matrix form. In the display device of the present invention, the light-emitting devices each possesses an emissive layer and a reflective element placed on the rear surface of the emissive layer; the emissive layer possesses at the said of the front side, a polarization separator which separates the light emitted from the emissive layer into two kinds of polarized components by the reflection and the transmission, and phase plate; the emissive layer substantially maintains the sate of the polarization of the light transmitted there-through; the reflective element at least reflects the circularly polarized light impinging in the vertical direction mainly as a circularly polarized light having a reverse helicity direction; and the polarization separator has a reflectance of the wavelength range from 520 nm to 600 nm smaller than a reflectance of range not more than 540 nm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device, which controls theoperations of light emitting devices for display. More particularly, theinvention relates to a technique available for light emitting devicessuch as organic light emitting diodes comprising a emissive layer havinga reflective element provided on the rear surface thereof and a displaydevice possessing such light emitting devices.

2. Description of the Related Arts

Organic light emitting diodes elements (devices) which emits a light byinjecting holes and electrons into a emissive layer to thereby convertan electric energy into a light energy. Such types of display devices(hereinafter sometimes abbreviated as “OLED display devices”), which isemission type ones, have a characterized to have a thin type and a lightweight unlike non-emissive type ones represented by liquid crystaldevices. Furthermore, OLED display devices are characterized to have awide viewing angle and have a rapid response time.

FIG. 22 is a schematic cross-sectional view showing one example of theconventional OLED display device. The OLED display device shown in thisfigure is composed of a transparent electrode 200 having a function ofan anode, a hole transporting layer 102, an emissive layer 100, anelectron transporting layer 101, and a reflective electrode 300comprising a light reflective metal serving as a cathode deposited on atransparent substrate 400 in this order. When direct current voltage isapplied between the transparent electrode 200 and the reflectiveelectrode 300, the holes, which have been injected from the transparentelectrode 200 arrive at the emissive layer 100 via the hole transportinglayer 102 and electrons injected from the reflective electrode 300arrive at the emissive layer 100 via the hole transporting layer 101,where the electrons and holes are recombined and the emission is broughtabout there-from.

Amongst lights emitted from the emissive layer 100, the lights 1000directing towards the transparent electrode 200 are passed through thetransparent electrode 200 and then are emitted from the transparentsubstrate 400. The lights 1001 directing towards the reflectiveelectrode 300 are reflected at the reflective electrode 300, then arepassed through the emissive layer 100, the transparent electrode 200 andthe like, and are similarly emitted from the transparent substrate 400.Consequently, in such a type of OLED display device, it is important forobtaining a bright image to use an electrode having a high reflectanceas the reflective electrode whereby the quantities of the lights emittedfrom the side of the transparent electrode is increased.

In such a configuration as described above, since the reflectiveelectrode is in a state of mirror having a high reflectance when theOLED display device is in the state where it emits no light, under abright environment, the image quality is deteriorated due to the factthat surrounding backgrounds are reflected in the reflective electrodeand the image which should be displayed in black is not becomes dark,reducing a contrast ratio. These lead to problems, which should besolved. As one means for solving such problems, a configuration has beenput into practical use in which a circular polarizer plate 800 is placedat the light emitting side of the transparent electrode 400. Thecircular polarizer plate 800 is composed of a polarizer plate 600 and aphase plate 700 serving as a quarter wave plate. The circular polarizerplate 800 is acted as follows:

An ambient light entering in the OLED display device from thecircumference is an un-polarized light as a rule. Upon passing theambient light through the polarizer plate 600, a linearly polarizedlight is transmitted through the polarizer plate 600, and a linearlypolarized light perpendicular to the light just mentioned is absorbedthereon. The linearly polarized light having been transmitted throughthe polarizer plate 600 has an influence of the phase plate 700 to becircularly polarized light (in this case, for example, dextrorotatorycircularly polarized light). Upon being reflected at the reflectiveelectrode 300, the circularly polarized light having been passed throughthe phase plate 700 becomes a circularly polarized light whose helicitydirection is reversed (levorotatory circularly polarized light). Thelight 2000R having been reflected at the reflective electrode 600 againenters in the phase plate 700, at which it has an influence of the phaseplate 700 at the time of passing through the phase plate 700 to beconverted into a linearly polarized light. In this case, the linearlypolarized light having been converted is absorbed on the polarizer plate600 and, thus, it is not returned to the external system. Specifically,the reflection of the ambient light on the reflective electrode 300 isreduced to darken the displaying of a black image, whereby the contrastratio is remarkably improved. Such a construction is described, forexample, in Japanese Patent Laid-Open Publication Nos. 8-509834 and9-127885, which are incorporated herein by references. However, the OLEDdisplay device having a circular polarizer plate is disadvantageous inthe fact that the displaying of the images are darkened since parts oflights emitting from the emissive layer are absorbed on the circularpolarizer plate. This is due to the fact that since the lights emittingfrom the emissive layer are generally un-polarized lights and, thus,approximately half of the light is are absorbed on the polarizer platemaking up the circular polarizer plate.

As a method for decreasing the lights absorbed on the polarizer plate torealize bright displaying, an OLED display device has been suggested,which has means for selectively reflecting circular polarized lightcomprising a cholesteric liquid crystal layer disposed between a quarterwave plate and a emissive layer. Such a construction is disclosed, forexample, in Japanese Patent Laid-Open Publication Nos. 2001-311826 and2001-357979, which are incorporated herein by references. In this case,the lights emitting from the emissive layer enter in the cholestericliquid crystal layer at which a specific circularly polarized lightcomponent is reflected, and a circularly polarized light componenthaving a helicity direction different from that of the former istransmitted. When being passed through the quarter wave plate, the lighthaving been transmitted through the cholesteric liquid crystal layer hasan influence of the quarter wave plate to be converted into a linearlypolarized light, which is transmitted through the polarizer plate.

On the other hand, the light reflected at the cholesteric liquid crystallayer is returned to the emissive layer and then reflected at thereflective electrode, at the time of this reflection, it becomes acircularly polarized light having a reverse helicity direction. Thelight reflected at the reflective electrode again enters in thecholesteric liquid crystal layer, at this time, it is passedthere-through and has an influence of the quarter wave plate to therebybe converted into a linearly polarized light, which is transmittedthrough the polarizer plate. Specifically, amongst the lights emittingfrom the emissive layer, the lights which are polarized light to beabsorbed on the polarizer plate are reflected at the cholesteric liquidcrystal layer, before they are absorbed on the polarizer plate, wherebythey are recycled. This obtains bright displaying of the images.

In the technique just mentioned, since lights which emit from theemissive layer and are transmitted through the polarizer plate, areincreased, much more bright displaying of the image can be obtained incomparison with the OLED display device only having a circularlypolarizer plate. However, in the case of using the later OLED displaydevice under a bright ambient condition, there arises the followingproblems associated with ambient lights, which will enter in the laterOLED display device: The ambient lights entering in the OLED displaydevice are generally un-polarized lights and at least halves of them areadsorbed on the polarizer plate, when they are passed through thepolarizer plate. When being transmitted through the quarter wave plate,the lights having been passed through the quarter wave plate have aninfluence thereof to be circularly polarized lights (for example,dextrorotatory circularly polarized light), and is transmitted throughthe cholesteric liquid crystal layer. Upon transmitting the lightshaving been passed through the cholesteric liquid crystal layer throughthe emissive layer while substantially maintaining their polarizedstates, and at the time of the reflection at the reflective electrode,they becomes circularly polarized lights whose helicity direction isreversed (levorotatory circularly polarized lights), and then reflectedagain when entering in the cholesteric liquid crystal layer.

Since the lights reflected at the cholesteric liquid crystal layer againreflected at the reflective electrode to be a circularly polarized lighthaving a reverse helicity direction (dextrorotatory circularly polarizedlight), the light at this time are transmitted through the cholestericliquid crystal layer, passed through the quarter wave plate and thepolarizer plate, whereby they exit out of the OLED display device. Thismeans that an unnecessary reflection of the ambient light is increasedby the arrangement of the cholesteric liquid crystal layer and, thus,indicates that the black image cannot be displayed in a sufficientmanner under a bright condition, leading to markedly decreasing of thecontrast ratio.

According to these prior arts described above, there is a descriptionthat in order to realize a wide wavelength range of selective reflectionwithin the visible wavelength range, a plurality of cholesteric liquidcrystal layers each having a different helical pitch are deposited. Asone embodiment of the prior art, the central wavelength of the selectivereflection at the cholesteric liquid crystal layers is set to be 550 nm,which is a high relative luminous efficiency in a photopic vision. Theseconditions are the conditions where the unnecessary reflection of theambient light brought about by placing the cholesteric liquid crystallayers becomes large, and thus, lead to a remarkable decrease in thecontrast ratio under a bright condition. Specifically, in the prior art,there is no description for the problem for increasing the reflection ofthe ambient light, which occurs in the case of the display device havingthe polarization separator such as the cholesteric liquid crystallayers, and no deal has been made.

As one method for realizing a full color display device using an organiclight-emitting diode, a method in which pixels corresponding to threeprimary colors (red (R), green (G), and blue (B)) are directly patternedhas been suggested. This method can be expected to realize a highefficiency by forming the pixels for respective colors under the optimumconditions. However, since the existing organic light-emitting diodeshave the wavelength of the light emission deviating from the desirablewavelength or since the distribution of the wavelength for lightemission is wide and gentle, no sufficient color reproduction can beobtained.

Also, since the luminous efficiency (1 m/W) is differed in the colors,the power consumption for displaying white becomes large. At the presentsituation, the organic light-emitting diode for green light emission hasthe highest luminous efficiency, but since the balance of chromaticityof each color is bad, it is required that the luminous intensity of theorganic light-emitting diode for green light emission, which has a highluminous efficiency is relatively decreased, and the luminousintensities of the organic light-emitting diodes for red and blue lightemission are increased, leading to decreased total efficiency.

The present invention has been done in light of the above situation, andan object of the present invention is to provide a display device whichcan realize bright display by effectively contributing the light emittedfrom the organic light-emitting diode to display, and which can realizedisplay with a high contrast even under a bright condition by decreasingthe reflection of the ambient light. Also, an object of the presentinvention is to provide a color display device, which shortens thedifference of the power in colors and enhances the efficiency. Anotherobjects will be apparent from the following description.

SUMMARY OF THE INVENTION

A display device of the present invention has light-emitting devicesmaking up a plurality of pixels placed in a matrix form. In the displaydevice of the present invention, the light-emitting devices eachpossesses an emissive layer and a reflective element placed on the rearsurface of the emissive layer; the emissive layer possesses at the frontside thereof, a polarization separator which separates the light emittedfrom the emissive layer into two kinds of polarized components by thereflection and the transmission, and phase plate; the emissive layersubstantially maintains the sate of the polarization of the lighttransmitted there-through; the reflective element at least reflects thecircularly polarized light impinging in the vertical direction mainly asa circularly polarized light having a reverse helicity direction; andthe polarization separator has a reflectance of the wavelength rangefrom 520 nm to 600 nm smaller than a reflectance of range not more than510 nm.

The polarization separator preferably has a reflection of a light havinga wavelength corresponding to blue higher than light having a wavelengthother than blue. Also, the polarization separator preferably has areflectance at a wavelength range of not more than 510 nm higher thanthat at other visible wavelength range.

The polarization separator preferably comprises a cholesteric liquidcrystal layer, and the phase plate comprises a quarter wave plate, andthe polarization separator, the phase plate, and the polarizer plate areprovided from the side of the emissive layer in this order.

Also, the polarization separator preferably comprises a cholestericliquid crystal layer substantially comprising one kind of a helicalpitch, and the central wavelength of the selective reflection is between400 nm to 490 nm.

In addition, it is preferred that the polarization separator isselectively formed on the position corresponding to the light-emittingdevice for blue light emission.

Also, it is preferred that the polarization separator comprises acholesteric liquid crystal layer substantially comprising one kind of ahelical pitch, the central wavelength of the selective reflection isbetween 400 nm to 490 nm, and the peak wavelength of the reflectionother than the main reflection range is between 510 nm to 600 nm.

In the display device of the present invention, an antireflection memberfor at least decreasing the reflection of the light having the mainwavelength range reflected by the polarization separator may be providedon a non-emissive area of the pixel composed of the light-emittingdevice.

Also, it is preferred that the polarization separator comprises aplurality of cholesteric liquid crystal layers each having a differenthelical pitch, and the central wavelength of the selective reflection isbetween 400 nm to 490 nm.

The polarization separator may comprise a plurality of cholestericliquid crystal layers each having a different helical pitch, and acholesteric liquid crystal layer having the central wavelength of theselective reflection between 400 nm to 490 nm amongst the plurality ofcholesteric liquid crystal layers has a thickness larger than thethickness of the layer, which has the maximum reflectance, the remainingcholesteric liquid crystal layers have a thickness smaller than thethickness of the layer, which has the maximum reflectance.

In this embodiment, the plurality of cholesteric liquid crystal layersmaking up the polarization separator may be stacked.

In a preferred embodiment of the display device according to the presentinvention, the plurality of cholesteric liquid crystal layers making upthe polarization separator are patterned in the direction of the innersurface of the substrate;

a cholesteric liquid crystal layer having a wavelength range of theselective reflection corresponding to a blue color is placed on theposition corresponding to the light-emitting device which emits a bluecolor;

a cholesteric liquid crystal layer having a wavelength range of theselective reflection corresponding to a green color is placed on theposition corresponding to the light-emitting device which emits a greencolor; and

a cholesteric liquid crystal layer having a wavelength range of theselective reflection corresponding to a red color is placed on theposition corresponding to the light-emitting device which emits a redcolor.

Also, the polarization separator may comprise a cholesteric liquidcrystal layer whose helical pitch is continuously changed, and thewavelength range which can obtain the maximum selective reflection bythe cholesteric liquid crystal layer is not more than 510 nm.

In another preferred embodiment of the display device of the presentinvention, the polarization separator is a linear polarizationseparator, which reflects a linearly polarized light having a prescribedwavelength range, and transmits lights other than the linearly polarizedlight having a prescribed wavelength range;

the phase plate comprises a quarter wave plate, and the polarizationseparator, the phase plate, and the polarizer plate are provided fromthe side of the emissive layer in this order.

In still another preferred embodiment of the display device of thepresent invention, the light-emitting devices comprises an organiclight-emitting diodes having an electrode also serving as the reflectiveelement, an emissive layer comprising organic thin films, and anoptional transparent electrode stacked with each other.

Furthermore, in the display device of the present invention, a spacesealed with a gas may be provided between the protective layer and thepolarization separator, and the distance between the space and theemissive layer is quarter the wavelength of the light emitted from theemissive layer or less.

In another aspect of the present invention, there is provided a displaydevice comprising a first substrate having a reflective electrode, anorganic emissive layer and an opposite electrode within the innersurface thereof in this order to make up a plurality of pixels placed ina matrix form, and a second substrate having a polarization separatorwithin the inner surface thereof opposite the inner surface of the firstsubstrate and having a phase plate and a polarizer plate on the outersurface thereof in this order, the polarization separator comprisingcholesteric liquid crystal layer, and the phase plate comprising aquarter wave plate.

In still another aspect of the present invention, there is provided adisplay device comprising a substrate having a reflective electrode, anorganic emissive layer and an opposite electrode within the innersurface thereof in this order to make up a plurality of pixels placed ina matrix form and having a polarization separator, a phase plate and apolarizer plate on the outer surface thereof in this order,

the polarization separator comprising cholesteric liquid crystal layer,and the phase plate comprising a quarter wave plate.

In these aspects, an active matrix elements for selecting and drivingthe pixel may be provided within the inner surface of the (first)substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cross-sectional view showing a schematicconfiguration for explaining the basic configuration and the operationprincipal of the display device according to the present invention.

FIG. 2 is a partially cross-sectional view showing a schematicconfiguration for explaining the basic configuration and the operationprincipal of the display device according to the present invention,which displays full color images.

FIG. 3 is a drawing showing one example of a spectral transmittance ofthe cholesteric liquid crystal layer making up the polarizationseparator.

FIG. 4 is a drawing which shows one example of a spectral reflectance ofthe display device according to the present invention in comparison withthe conventional technique.

FIG. 5 is a drawing showing one example of the light emitting spectra ofthe conventional display device.

FIG. 6 is a drawing showing one example of the light emitting spectraaccording to the present invention.

FIG. 7 shows one example of chromaticity coordinates.

FIG. 8 is a block diagram schematically showing the layout of the wholeof the OLED display device according an embodiment of the presentinvention.

FIG. 9 shows an equivalent circuit of the active matrix constituted in adisplay portion.

FIG. 10 is a partially cross-sectional view showing a schematicconfiguration for explaining the basic configuration the OLED displaydevice according to the present invention.

FIG. 11 is a partially cross-sectional view showing a schematicconfiguration for explaining the basic configuration the OLED displaydevice according to the present invention, which displays full colorimages.

FIG. 12 is an explanatory drawing showing one embodiment of the displayoperation of the OLED display device according to the present invention.

FIG. 13 is an explanatory drawing showing one embodiment of the displayoperation of the OLED display device according to the present invention.

FIG. 14 is a partially cross-sectional view showing a schematicconfiguration of another embodiment of the OLED display device accordingto the present invention.

FIG. 15 is a partial plane view schematically showing the configurationof the pixel portion of the OLED display device according to anotherembodiment of the present invention viewing from the first substrate.

FIG. 16 is a partial cross-sectional view showing one example of theconfiguration of the storage capacitor in another embodiment of the OLEDdisplay device according to the present invention.

FIG. 17 is a partial cross-sectional view showing the basicconfiguration of the OLED display device, which displays full colors,according to another embodiment of the present invention.

FIG. 18 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to another embodimentof the present invention.

FIG. 19 is a drawing showing one example of a spectral transmittance ofthe cholesteric liquid crystal layer making up the polarizationseparator in still another embodiment of the present invention.

FIG. 20 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to still anotherembodiment of the present invention.

FIG. 21 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to still anotherembodiment of the present invention.

FIG. 22 is a partial cross-sectional view showing the basicconfiguration of the conventional OLED display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described by referring to thedrawings. FIG. 1 is a partially cross-sectional view showing a schematicconfiguration for explaining the basic configuration and the operationprincipal of the display device according to the present invention. FIG.2 is a partially cross-sectional view showing a schematic configurationfor explaining the basic configuration and the operation principal ofthe display device according to the present invention, which displaysfull color images.

First, referring to FIG. 1, the basic configuration and the operationprincipal of the display device according to the present invention willbe described.

A light emitting device part according to the display device of thepresent invention is composed of an organic light emitting diode 24comprising a transparent electrode 200 serving as an anode formed on thesubstrate (not shown), a reflective electrode 300 serving as a cathodeand as a specular reflector, an organic layer 110 formed between theanode and the cathode, and a polarization separator 500, a phase plate700, and a polarizer plate 600 disposed in this order from the frontsurface side of the light emitting diode 24 (the side of the transparentelectrode 200).

The organic layer 110 making up the organic light emitting diode 24 maybe a laminate comprising, from the cathode side (the side reflectiveelectrode 300) to the anode (the side of the transparent electrode 200),an electron-transporting layer 101, an emissive layer 100, and ahole-transporting layer 102. The emissive layer 100 and theelectron-transporting layer 101 may be a mono-layer by utilizing amaterial which can make up both layers. As the configuration of thelight emitting diode, one which has a configuration of an anode bufferlayer and/or a hole injecting layer may be used. An electrode material,which has a high work function and which is a transparent, may beutilized as the anode (transparent electrode 200, and, for example, ITO(indium tin oxide) may be suitably used. Also, IZO (indium zinc oxide)may be utilized.

As the reflective electrode 300 which is the cathode 300, Al, Mg, Mg—Alalloy, Al—Li alloy, and the like which have a low work function, may beused. The sole use of Al requires a high driving voltage and leads to ashortened life, and, thus, a very thin Li compound such as lithium oxideLi₂O or lithium fluoride LiF is inserted between the Al film and theorganic layer to obtain characteristics equivalent to Al—Li alloy. Also,it is possible to dope a portion of the organic layer in contact withthe cathode with a highly reactive metal such as lithium or strontium tolower the driving voltage. From the viewpoint of the utilizationefficiency of the light emitting from the emissive layer, the reflectiveelectrode 300 is preferably made of a material having a highreflectivity. Furthermore, from the reason, which will be describedlater on, the reflective electrode 300 preferably possesses a specularreflector, which at least reflects a circularly polarized lightimpinging from the vertical direction as a circularly polarized lighthaving a reverse helicity direction from the viewpoints of thedecreasing of the reflection of the ambient light and of the utilizationefficiency of the light emitting from the emissive layer.

As the material of the organic layer 110, a material is used, whichemits a light on a desired color when a prescribed voltage is appliedbetween the anode (transparent electrode 200) and the cathode(reflective electrode 300). Examples of red light-emitting materialswhich may be used for the hole transporting layer 102, include, but arenot restricted to, alpha-NPD(N,N′-di(alpha-naphtyl)-N,N′-diphenyl-1,1′-bisiphenyl-4,4′-diamine)) andtriphenyldiamine derivatives such as TPD(N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine. An Example of redlight emitting materials which may be used for the electron-transportinglayer (used both for the electron-transporting layer and the emissivelayer) includes, but is not restricted to, Alq3(tris(8-quinolinolate))aluminum having DCM-1(4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-2-4H-pyrandispersed therein.

Examples of green light-emitting materials which may be used for thehole transporting layer 102, include, but are not restricted to,alpha-NPD and triphenyldiamine derivatives such as TPD, and examples ofgreen light emitting materials which may be used for theelectron-transporting layer (used both for the electron-transportinglayer and the emissive layer) include, but is not restricted to, Alq,Bebq (bis (8-hydroxyquinolinate)-beryllium and Alq having been dopedwith quinacridone.

Examples of blue light emitting materials which may be used for the holetransporting layer 102, include, but are not restricted to, alpha-NPDand triphenyldiamine derivatives such as TPD, examples of blue lightemitting materials which may be used for the emissive layer 100 include,but are not restricted to, DPVB1 (4,4′-bis(2,2-diphenylvinyl)biphenyl)or a mixture of DPVBi with BczVBi(4,4′-bis(2-carbazolevinylene)biphenyl) doped materials comprisingdistyrylallylene derivatives as hosts and distyrylamine derivatives asguest. Alq3 may be used as the material for the electron-transportinglayer 101 for the blue light emitting material. Zn(oxz)2 (zinc complexof 2-(o-hydroxylphenyl)-benzoxazple) may be used as the blue lightemitting materials for the electron-transporting layer (used both forthe electron-transporting layer and the emissive layer).

On the other hand, in addition to a low molecular material, polymericmaterial may also be used. For example, a stacked layer comprisingPEDT/PSS (a mixed layer of polyethylene dioxythiophene and polystyrenesulphonate)and PPV (poly(p-phenylene vinylene) can be used as thehole-transporting layer 102 and the emissive layer 100. In this case,although no electron-transporting layer is provided, it may be providedas occasion may demand.

The green light emission is realized by formulation of green ink intoPPV, the emitting of the red light is realized by the formation of greenink together with Rhodamine 101 as a red light emitting dopant. As aemissive layer, which emits a blue light can be used F8(poly(dioctylfluorene). Also, as the polymeric materials other thanthose described previously, pigment-containing polymers such as PVK(polyvinyl carbazole) may be used. In any case, each layer making up theorganic layer 110 is thin, which is approximately sever 1 tennanometers, the polarization states of the lights which are passedthrough each layer are substantially maintained.

In the organic light-emitting diode 24 configured as described above, adirect current power source is connected to the transparent electrode200, which is the anode, and the reflective electrode 300, which is thecathode, and when a direct current voltage is applied between thetransparent electrode 200 and the reflective electrode 300, the holesinjected from the transparent electrode 200 arrive at the emissive layervia the hole-transporting layer 102 and the electrons injected from thereflective electrode 300 arrive at the emissive layer via theelectron-transporting layer 101, respectively, to recombine theelectron-hole whereby a light having a prescribed wavelength is emitted.

Opposite the organic layer 110, on the transparent electrode 200 arestacked a polarization separator 500, a phase plate 700, and a polarizerplate 600 are stacked in this order. The polarization separator 500 hasa function that a light having a prescribed wavelength range isseparated into two light components having complementary states of thepolarization by the reflection and the transmission. As the polarizationseparator 500 intended herein is suitably a cholesteric liquid crystallayer.

Since the cholesteric liquid crystal layer has specific opticalcharacteristics based upon a helical molecular alignment, the lightsimpinging parallel to the helical axis show selective reflection that atthe wavelength corresponding to the pitch of the cholesteric helix, onecircularly polarized light component having a first circular helicity isreflected, and the other is transmitted. When the central wavelength ofthe selective reflection through the cholesteric liquid crystal layer istaken as λ₀ and the wavelength range thereof is taken as Δλ, they arerepresented by the following formulae (1) and (2)λ₀ =n _(m) ·p   (1)Δλ=Δn·p   (2)where p is a helical pitch of the cholesteric liquid crystal layer,n_(m) is an average refractive index. When n_(e) and n₀ are theextraordinary and ordinary refractive indices, respectively, n_(m) andΔn are represented by the following formulae (3) and (4)n _(m)=√{square root over (((n _(e) ² +n ₀ ²)/2))}  (3)Δn=n _(e) −n ₀   (4)

As the cholesteric liquid crystal layer, preference is given to use apolymerized cholesteric liquid crystal film. For example, a filmproduced, for example, by forming an alignment layer such as polyvinylalcohol on a triacetyl cellulose film to subject an alignment treatment,and forming a cholesteric liquid crystal film thereon.

Here, when the display device which can display full colors is realized,the wavelength of the light emitted from the organic light emittingdiode should correspond to the primary colors, red, green and blue pereach pixel. Specifically, as outlined in FIG. 2, the organic layer 110is configured to be patterned light emitting organic layers for eachprimary colors, i.e., a red light emitting organic layer, a green lightemitting organic layer, and a blue light emitting organic layercorresponding to a red light emitting portion 25R, a green lightemitting portion 25G and a blue light emitting portion 25B, and thepeaks of the emitting light of the organic light emitting diode 24 arediffered depending upon pixels. In contrast, the wavelength range of theselective reflection at the cholesteric liquid crystal layer or thecentral wavelength λ₀ of the selective reflection corresponds to thelight emitting wavelength range or the peak wavelength of the lightemission of the organic light emitting diode 24 of the blue lightemitting pixel.

In the case where the light emitting from the organic light emittingdiode of the blue light emitting pixel is not enough for blue light, itis desirable that the wavelength range of the selective reflection ofthe cholesteric liquid crystal layer and the central wavelength of theselective reflection are set at the shorter wavelength range than thoseof the peak wavelength of the light emission from the organic lightemitting diode and wavelength range of the light emission. Specifically,the central wavelength of the selective reflection is preferably from400 nm to 490 nm, and preferably from 420 nm to 480 nm, and thewavelength range of main selective reflection is desirably not more than510 nm. As described fully later on, this is for the purpose ofminimizing the reflection of the ambient light, and for effectivelyutilizing an effective light as a blue light to increase the colorpurity of the blue whereby the total efficiency of the display device isimproved.

FIG. 3 is a drawing showing one example of a spectral transmittance ofthe cholesteric liquid crystal layer making up the polarizationseparator, and specifically shows one example of spectral transmittanceof the cholesteric liquid crystal layer having a selective reflectioncorresponding to the blue light as described above. FIG. 3 shows thewavelength-dependency of the transmittance when an un-polarized lightenters in the cholesteric liquid crystal layer. The phase plate 700 andthe polarizer plate 600 correspond to those making up the circularlypolarized plate in the prior art. Specifically, the polarizer plate 600transmits a specific linearly polarized light amongst the lights passingthere-through, and absorbs a linearly polarized light perpendicular tothe former. The phase plate 700 is made up of the material serving as aquarter wave plate which converts the linearly polarized light passingthrough the polarizer plate 600 into a substantially circularlypolarized light.

The polarizer plate 600 which can be used is one which is prepared byapplying protective layers made of triacetyl cellulose on both surfacesof a film having a polarization function imparted by absorbing iodine ona stretched polyvinyl alcohol film. As the phase plate 700, atransparent, uniaxial stretched polymer films such as made of polyvinylalcohol, polycarbonate, polysulfoner polystyrene, and polyarylate can beused. Since the polymer film making up the phase plate 700 haswavelength-dependency of the refractive index as a rule, no sufficientperformance can sometimes be obtained, when the phase plate 700 is madeof one polymer film with respect to lights having a wide wavelengthrange. For this reason, retardation films each having a different phasedifference may be stacked with slanting their slow axes to constitute aphase plate is reflected, and the components other than the formercomponent are transmitted.

Specifically, in the blue light corresponding to the wavelength range ofthe selective reflection at the cholesteric liquid crystal layer 500, acircularly polarized light component having one helicity direction (forexample, levorotatory circularly polarized light) is reflected, and acircularly polarized light component having reverse helicity directionto the former (dextrorotatory circularly polarized light) istransmitted. Also, almost all parts of the red light and the green lightare transmitted through the cholesteric liquid crystal layer 500.Amongst the light 1002 having been transmitted through the polarizationseparatir 500, the light corresponding to the wavelength range of theselective reflection at the cholesteric liquid crystal layer isconverted into a linearly polarized light, which is transmitted throughthe polarization separator 600, by the action of the polarizer plate600, and the light transmitted through the polarizer plate 600 isdirected towards the side of the viewer 10000. Also, amongst the light1002, approximately half of the light not corresponding to thewavelength range of the selective reflection at the cholesteric liquidcrystal layer is absorbed on the polarizer plate 600, and the remaininghalf is directed towards the side of the viewer 10000.

On the other hand, the light 1003 reflected at the polarizationseparator 500 is transmitted through the emissive serving as a quarterwave plate within a wide wavelength range. The direction of the slowaxis of the phase plate 700 is decided so that the circularly polarizedlight passing through the polarizer plate 600 and the phase plate 700becomes a circularly polarized light having a reverse the helicitydirection (e.g., dextrorotatory circularly polarized light) to thecircularly polarized light which is selectively reflected by thecholesteric liquid crystal layer making up the polarization separator500 (levorotatory circularly polarized light).

Subsequently, the operation of the display device according to thepresent invention will now be described by referring to FIG. 1 and FIG.2. When a direct current power source is connected to the transparentelectrode 200 and the reflective electrode 300, and current is run, alight with a prescribed wavelength is emitted from the emissive layer100. The light 1000 (1000R, 1000 G, and 1000 B in FIG. 2) emitted fromthe emissive layer 100 is directed towards the transparent electrode 200directly or after it is reflected at the reflective electrode 300, it istransmitted through the transparent electrode 200 and then enters in thepolarization separator 500. At this time, since the light emitted fromthe emissive layer 100 is un-polarized, amongst the light components, acircularly polarized light component having one helicity direction (forexample, levorotatory circularly polarized light) corresponding to thewavelength range of the selective reflection at the cholesteric liquidcrystal layer layer and the like while substantially maintaining itspolarization states, then reflected at the reflective electrode 300, andis again directed towards the polarization separator 500. At the time ofthe reflection at the reflective electrode 300, the helicity directionof the light 1003 is reserved (i.e., the light 1003 becomes a circularlypolarized light having a reverse helicity direction such as adextrorotatory circularly polarized light) and, thus, at this time, thelight 1003 is transmitted through the polarization separator 500. Thelight 1003 transmitted through the polarization separator 500 isconverted into a linearly polarized light, which is transmitted throughthe polarizer plate 600 by the action of the phase plate 700, and thentransmitted through the polarizer plate 600 to be directed towards theside of the viewer 10000.

Consequently, amongst the light emitting from the emissive layer 100,almost all parts of the light corresponding to the wavelength range ofthe selective reflection by the cholesteric liquid crystal layer aredirected towards the side of the viewer 10000 without being absorbed onthe polarizer plate 600. Specifically, amongst the lights which areabsorbed on the polarizer plate to be useless in conventional, the lightcorresponding to the blue color, which is corresponds to the wavelengthrange of the selective reflection by the cholesteric liquid crystallayer is reflected at the polarization separator (cholesteric liquidcrystal layer) can be reused, leading to the advantage in terms of beingbrightness.

Subsequently, the ambient light, which enters in the display device fromthe circumferences under bright conditions will now be described. Theambient light 3000 entering in the display device from thecircumferences is generally un-polarized. Amongst the ambient light3000, when being passed through the polarizer plate 600, a prescribedlinearly polarized light is absorbed, and the linearly polarized lightperpendicular thereto is transmitted. The linearly polarized lighthaving been transmitted through polarizer plate 600, by the action ofthe phase plate 700 to be a circularly polarized light (for example,dextrorotatory circularly polarized light) The light having been passedthrough the phase plate 700 is transmitted through the polarizationseparator 500, and becomes a circularly polarized light having a reversehelicity direction (levorotatory circularly polarized light) as a resultat the time of being reflected at the reflective electrode 300. Thelight reflected at the reflective electrode 300 enters in thepolarization separator 500 at which a light 3001 having a wavelengthother than the wavelength range of the selective reflection at thecholesteric liquid crystal layer making up the polarization separator500 is transmitted through the polarization separator 500 as is, thelight having a wavelength corresponding to the wavelength range of theselective reflection at the cholesteric liquid crystal layer isreflected. The light 3001 having been transmitted through thepolarization separator 500 becomes a linearly polarized light which isabsorbed on the polarizer plate 600 by the action of the polarizer plate600, and is absorbed on the phase plate 700; thus, it is not returned tothe external display device.

On the other hand, the light 3002 reflected at the polarizationseparator 500 is reflected at the reflective electrode 300 and, is againdirected to the polarization separator 500. At the time of reflection atthe reflective electrode 300, the light 3002 becomes a circularlypolarized light whose helicity direction of the light is reversed and,thus, the light 3002 is transmitted through the polarization separator500 at this time. The light 3002 having been transmitted through thepolarization separator 500 is converted into a linearly polarized lightwhich is transmitted through the polarizer plate 600, by the action ofthe phase plate 700, and is transmitted through the polarizer plate 600to be directed towards the side of the viewer 10000. Specifically, atleast half of the ambient light 300 entering in the display device isfirst absorbed on the polarizer plate 600. The light having beentransmitted through the polarizer plate 600 is reflected at thereflective electrode 300 and then enters in the polarization separator500, and the light 3001, which is transmitted through the polarizationseparator 500, is absorbed on the polarizer plate 600. For this reason,the light returning to the external display device is only slight amountof the light 3002 corresponding to the wavelength range of the selectivereflection at the cholesteric liquid crystal layer.

FIG. 4 is a drawing which shows one example of a spectral reflectance ofthe display device according to the present invention in comparison withthe conventional technique, and specifically shows one example of thespectral reflectance of the OLED display device using the cholestericliquid crystal layer having characteristics exemplified in FIG. 3. Forcomparison, FIG. 4 shows the reflectance of the display device in thecase where a plurality of cholesteric liquid crystal layers each havinga different helical pitch are stacked as the polarization separator inorder to realize a wide wavelength range of the selective reflectioncover the visible wavelength range.

As shown in FIG. 4, in the case of the conventional technique where aplurality of cholesteric liquid crystal layers each having a differenthelical pitch are stacked as the polarization separator, a spectralreflectance is heightened over a wide wavelength range and the luminousreflectance becomes as high as 20%. In contrast, in the case where thewavelength range for main selective reflection is set to the wavelengthrange corresponding to the blue light, the wavelength range having ahigh spectral reflectance becomes only a light corresponding to thewavelength range of the selective reflectance of the cholesteric liquidcrystal layer, and the luminous reflectance becomes 5%, which is quarterof the conventional technique. This indicates that under the samebrightness of displaying, the contrast ratio of the display deviceaccording to the present invention under the ambient light (in brightenvironment) is four times that of the conventional technique.

It is noted that this reflectance is a value containing the surfacereflectance of the polarizer plate, which is 4%, and considering thatanti-reflective coating made of multilayers is formed on the polarizerplate, the reflectance of the present invention is very small, which istenth the conventional technique and, thus, the contrast ratio of thedisplay device according to the present invention under the ambientlight is ten times that of the conventional technique. Specifically, thedisplay device according to the present invention can realize thedisplaying of dark black image even in bright environment because ofdecreased reflection of the ambient light, and the contrast ratio can beincreased.

Here, for suppressing the reflection of the ambient light, it isimportant, for allowing a human to feel that an unnecessary reflectionis small, to reduce the reflection of a green light, which has a highrelative luminous efficiency in a photopic vision, i.e., a light havinga wavelength of from approximately 520 to 600 nm. For this reason,according to the present invention, the reflection of the ambient lightis suppressed by making a wavelength range of the main selectivereflection of the cholesteric liquid crystal layer narrow so as tobecome a part of the visible wavelength range, and setting thewavelength range of the main selective reflection of he cholestericliquid crystal layer to be blue color, which is a low relative luminousefficiency in a photopic vision. Specifically, even if the wavelengthrange of the selective reflection is narrower than the visiblewavelength range, when the central wavelength of the selectivereflection resides around 555 nm, which is a high relative luminousefficiency in a photopic vision, the reflectance become high so that thecontrast ratio under an ambient light is remarkably decreased. Incontrast, if the central wavelength of the selective reflection is blue(wavelength from 450 nm to 480 nm) or red (wavelength from 640 nm to680) and the reflection of a light having a wavelength from 520 nm to600 nm, which is a high relative luminous efficiency in a photopicvision, is decreased, then the luminous reflectance becomes small, andan observer feels that unnecessary reflection is small.

Subsequently, improvement of color purity and improvement of efficiencywill now be described. As shown in FIG. 2, amongst the light emittedfrom the emissive layer, almost all parts of the red light 100OR and thegreen light 1000G are transmitted through the cholesteric liquid crystallayer, which is the polarization separator 500, and approximately halfof them is absorbed on the polarizer plate, and the remaining half isemitted to the side of the viewer 10000. On the other hand, amongst thelight emitted from the emissive layer, almost all parts of thewavelength range of the blue light 1000B are overlapped with thewavelength range of the selective reflection of the cholesteric liquidcrystal layer. For this reason, amongst the blue light, the light 1002,which corresponds to the wavelength range of the selective reflection ofthe cholesteric liquid crystal layer and is transmitted through thecholesteric liquid crystal layer is converted into a linearly polarizedlight, which is transmitted through the polarizer plate 600 by theaction of the phase plate 700 and is transmitted through the polarizerplate 600 to be directed towards the side of the viewer 10000.

Also, amongst the blue light 1000B, the light 1003 having been reflectedat the polarization separator 500 is transmitted through the emissivelayer etc. while substantially maintaining its polarization states, thenreflected at the reflective electrode 300, and is again directed towardsthe polarization separator 500. At the time of the reflection at thereflective electrode 300, the helicity direction of the light 1003 isreserved (i.e., the light 1003 becomes a circularly polarized lighthaving a reverse helicity direction such as a dextrorotatory circularlypolarized light) and, thus, at this time, the light 1003 is transmittedthrough the polarization separator 500. The light 1003 transmittedthrough the polarization separator 500 is converted into a linearlypolarized light, which is transmitted through the polarizer plate 600 bythe action of the phase plate 700, and then transmitted through thepolarizer plate 600 to be directed towards the side of the viewer 10000.

Consequently, amongst the light emitting from the emissive layer 100,almost all parts of the light corresponding to the wavelength range ofthe selective reflection by the cholesteric liquid crystal layer aredirected towards the side of the viewer 10000 without being absorbed onthe polarizer plate 600. Specifically, amongst the lights which areabsorbed on the polarizer plate to be useless in conventional, the lightcorresponding to the blue color, which is corresponds to the wavelengthrange of the selective reflection by the cholesteric liquid crystallayer is reflected at the polarization separator (cholesteric liquidcrystal layer) can be reused, leading to the advantage in terms of beingbrightness.

Here, as shown in FIG. 3, the wavelength distribution of the selectivereflection of the cholesteric liquid crystal layer making up thepolarization separator 500 is generally a sharp distribution. Asdescribed above, the wavelength range of the selective reflection ofcholesteric liquid crystal layer can make narrower than the wavelengthrange of the organic light-emitting diode by selecting Δn and thehelical pitch p. Also, even in the case of the same light-emitting peakwavelength, when the light-emitting wavelength range is wide and is ofgentle distribution, the light becomes a color, which is a low colorpurity (here, excitation purity: the ratio of the distance from thewhile light source in a chromaticity diagram) and is pale.

Consequently, if the wavelength range of the light which is reflected atthe polarization separator 500 to be reused is set to be narrower peakband than that of the light emitting wavelength range of the emissivelayer, the wavelength distribution of the light emitted from the displaydevice in real becomes narrower distribution than the light emitted fromthe emissive layer and, thus, the excitation purity can be heightened.Specifically, in the display device according to the present invention,the excitation purity can be heightened to the light reflected at thepolarization separator 500 to be reused relative to the excitationpurity of the light emission by the organic light-emitting diode 24itself. What is more, as described above, since the reflection of theambient light becomes small when the wavelength range of the selectivereflection of the cholesteric liquid crystal layer making up thepolarization separator 500 is narrow, there is an advantage that muchhigher contrast ratio can be obtained under a bright condition.

FIG. 5 is a drawing showing one example of the light emitting spectrumof the conventional display device; shows one example of a lightemitting spectrum of the red light emitting pixel, one example of alight emitting spectrum of the green light emitting pixel, and oneexample of a light emitting spectrum of the blue light emitting pixel,when the OLED display device only possessing a circular polarizer plateis observed from the front side; and is a graph showing thewavelength-dependency of a relative value of the light emittingintensity (W/m²/sr) of each organic color light emitting diode. Thegraph shown in FIG. 5 indicates the case where a white image isdisplayed in which an x,y-chromaticity coordinates (x,y)=(0.3100,0.3300) in CIE 1931 chromaticity diagram. In this figure, the linesshown as blue, green, red, and white show respective color lightintensity as a normalized value (relative value) at the maximumintensity of the green light, when the OLED display device is observedfrom the front side.

FIG. 6 is a drawing showing one example of the light emitting spectraaccording to the present invention; shows the wavelength-dependency ofrelative intensity of each emitted color when in the OLED display devicecomposed of an organic light emitting diode having the same lightemitting spectrum as that shown in FIG. 5, the cholesteric liquidcrystal layer shown in FIG. 3 is used as the polarization separator 500.Similar to FIG. 5, FIG. 6 shows the case where a white image isdisplayed in which an x,y-chromaticity coordinates (x,y)=(0.3100,0.3300) in CIE 1931 chromaticity diagram. In this figure, the linesshown as blue, green, red, and white show respective color lightintensity as a normalized value (relative value) at the maximumintensity of the green light, when the OLED display device is observedfrom the front side. For reference, the case where no polarizationseparator is used at the same light emitting intensity, i.e., the casewhere only the circular polarizer plate is used are depicted as (BLUE)and (GREEN). With respect to red, the difference due to the presence orabsence of the polarization separator is small, and thus, it is notdepicted.

FIG. 7 shows one example of chromaticity coordinates showing anx,y-chromaticity coordinates in CIE 1931 chromaticity diagram when red,blue and green each is displayed as a single color in the OLED displaydevice having a light emitting spectrum exemplified in FIG. 6. Forcomparison, FIG. 7 also shows an x,y-chromaticity coordinates of theconventional OLED display device only having a circular polarizer plate.

As for the conventional OLED display device where a pluralitycholesteric liquid crystal layers each having a different helical pitchare deposited, or where a cholesteric liquid crystal layer whose helicalpitch is continuously changed is used to realize a wide wavelength rangeof the selective reflection over a full visible wavelength range,although the absolute value of the intensity become large, the relativeintensity thereof shown in FIG. 5 and the x,y-chromaticity coordinatesthereof shown in FIG. 7 may be considered to be similar. As exemplifiedin FIG. 5, the organic light emitting diode at the present situationcannot give sufficient color reproductivity range shown in FIG. 7, sinceits central wavelength of emitting light is different from a desiredwavelength or the distribution of the light emitting wavelength is wideand gentle.

For example, when a white color whose chromaticity coordinates(x,y)=(0.3100, 0.3300), since the balance of the chromaticitycoordinates of each light emitting color is bad, it is required todecrease the light emitting intensity of the organic light-emittingdiode for green light emission, and to increase the light emittingintensity of the organic light-emitting diode for red light emission andthat of the organic light-emitting diode for blue light emission. Here,the luminous efficiency of the organic light-emitting diode for greenlight emission is higher than that of the organic light-emitting diodefor red light emission and that of the organic light-emitting diode forblue light emission. For this reason, in the case of displaying a whitecolor, the emission intensity of the organic light-emitting diode forgreen light emission, which has high efficiency, is decreased, and theemission intensities of the organic light-emitting diode for red lightemission and that of for blue light emission, which has low efficiency,are relatively increased, whereby the total efficiency of the displaydevice is decreased.

Furthermore, the necessary of increasing the emission intensities ofblue and red colors leads to the fact that in the case of displayingwhite color, the power of the organic light-emitting diode for bluecolor or for red color becomes larger than that of the organiclight-emitting diode for green color, and thus, the power consumption isdiffered in the colors. For example, in the case of displaying a whitecolor with a luminance of 100 cd/m², considering that efficiency of eachcolor, the ratio of the power consumption of the organic light-emittingdiodes for emitting red (R), green (G), and blue (B) colors (R:G:B)becomes 5.04:1.00:2.81, indicating that depending upon the colors, themaximum power difference as much as five times occurs.

In contrast, as shown in FIG. 6, according to the present invention, byreusing the light within the wavelength range effective for blue light,which has conventionally been absorbed on the polarizer plate, theintensity of the light corresponding to the blue light is increased.Specifically, the light emission spectrum depicted as BLUE in thisfigure can be realized at the light-emission spectrum shown as (BLUE) inthe case of the conventional OLED display device only having thecircular polarizer plate. For this reason, with regard to the singlecolor of blue, according to the OLED display device according to thisembodiment, the maximum intensity increases 1.77 times, and theluminance increases 1.27 times the conventional OLED display device onlyhaving the circular polarizer plate. Furthermore, whereas thex,y-chromaticity coordinates (x,y) is (0.1413, 0.1899) in theconventional OLED display device, the x,y-chromaticity coordinates (x,y)in the present invention is (0.1370, 0.1486), indicating that theexcitation purity is increased from 75.4% to 82.3% to widen a colorgamut.

Also, for example, in the case where the white color whose chromaticitycoordinates (x,y) is (0.3100, 0.3300), and the luminance is 100 cd/m²,the ratio of the power consumption of the organic light-emitting diodesfor emitting red (R), green (G), and blue (B) colors (R:G:B) becomes3.95:1.00:1.10, indicating that difference of the power consumption incolors is decreased and the power consumption of the green color andthat of the blue color are substantially equal to each other. Moreover,by the fact that the light emission intensity of the organiclight-emitting diode for green light, which has a high luminousefficiency, is relatively increased, the power consumption fordisplaying a white color is decreased and becomes approximately 84% incomparison with the conventional OLED display device only having thecircular polarizer plate.

It is noted that as exemplified in FIG. 3, the selective reflection atthe cholesteric liquid crystal layer has a purity of minor reflectivewavelength ranges in addition to the main reflective wavelength range.Since these minor reflective wavelength ranges contribute to theenhancement of the luminance, for example, one of the minor reflectivewavelength ranges must be accorded with the peak wavelength of theorganic light-emitting diode for green which has a high relativeluminous efficiency in a photopic vision. This contributes to theenhancement of the luminance and the total efficiency of the displaydevice. In this embodiment, the light emission spectrum shown as GREENin FIG. 6 can be realized in the light emission spectrum shown as(GREEN) in the case of the conventional OLED display device only havinga circularly polarizer plate, indicating that the luminescence isincreased 6%. Since the reflection of the wavelength ranges other thanthe main wavelength range of the selective reflection are small, theincreasing of the reflection of the ambient light becomes small, whichwould not lead to serious problem.

It is noted that while the case where the wavelength range of theselective reflection of the cholesteric liquid crystal layer is set tobe blue has been described, the present invention is not intended toexclude the case where the wavelength range of the selective reflectionof the cholesteric liquid crystal layer is set to be red in terms ofavoiding a high relative luminous efficiency in a photopic vision forthe purpose of suppressing the reflection of the ambient light. In thiscase, the chromaticity of the red can be improved and the luminescencecan be enhanced to decrease the total power consumption of the displaydevice. However, in the case of considering the viewingangle-dependency, it is desired that the wavelength range of theselective reflection of the cholesteric liquid crystal layer is set tobe blue.

Here, the wavelength range of the selective reflection of thecholesteric liquid crystal layer is changed depending upon an incidentangle of the light. Specifically, if the incident angle of the light isincreased, the wavelength range of the selective reflection is sifted tothe side of short wavelength. For this reason, in the case where thewavelength range of the selective reflection corresponds to a red color,the wavelength range of the selective reflection is shifted towards thegreen side, which has a high relative luminous efficiency in a photopicvision, if the incident angle of the light is increased to increase theluminous reflectance. Conversely, in the case where the wavelength rangeof the selective reflection corresponds to a blue color, the wavelengthrange of the selective reflection is shifted towards a ultravioletranger which has a low relative luminous efficiency in a photopicvision. This does not lead to any problem because of ultraviolet rangeis difficult to be viewed.

A high luminous efficiency is expected in a phosphorescentorganomatallic materials, which are said to utilize phosophorescence,and at the present situation, there are materials for obtaining a highluminous efficiency in the green light emission and the red lightemission. However, from now on, there is no material for obtaining ahigh luminous efficiency in the blue light emission like that forobtaining a high luminous efficiency in the green light emission and thered light emission. Consequently, with regard to the green lightemission and the red light emission, phosphorescent organomatallicmaterials are used, and with regard to the blue light emission, afluorescent material is used and the polarization separator having thereflective wavelength range in a blue light is used to enhance a bluelight. Such a configuration as just mentioned realize a display devicehaving well-balance efficiency for primary colors and high luminousefficiency.

Subsequently, an embodiment of the OLED display device which is drivenby an active matrix will now be described by referring to the drawings.FIG. 8 is a block diagram schematically showing the layout of the wholeof the OLED display device according an embodiment of the presentinvention, and FIG. 9 shows an equivalent circuit of the active matrixconstituted in a display portion. In FIG. 8 and FIG. 9, referentialnumber 1 indicates an OLED display device, and 2 indicates a displayportion thereof. As shown in FIG. 8, the display portion 2 is providedon approximately center of a substrate 6 of the OLED display device. Inthis figure, a data driving circuit 3 which outputs an image signal to adata line 7 is provided on an upper portion of the display portion 2,and a scan driving circuit 4 which outputs a scan signal to a gate lineis provided on a left side of the display portion 2. These drivingcircuits 3 and 4 are composed of a shift register circuit, a levelshifter circuit, analog switching circuit and so on comprisingcomplementary type circuit due to N-channel type TFT (thin filmtransistor) and P-channel TFT.

Similar to the active matrix type liquid crystal display device, on thedisplay device 1, a plurality of gate lines and a plurality of datalines extending to the direction crossing to the direction of theextension of the gate lines are provided. As shown in FIG. 9, pixels 20in a matrix state are placed at portions where these gate lines G1, G2,. . . Gm and these data lines D1, D2, . . . Dn are crossed to eachother. Each pixel is composed of an organic light-emitting diode 24, astorage capacitor 23, a switching transistor 21 comprising an N-channeltype TFT where a gate electrode are connected to the gate line, one ofsource/drain electrodes is connected to the data line, and the other isconnected to the storage capacitor 23, and a driving transistor 21comprising an N-channel type TFT where the gate electrode is connectedto the storage capacitor 23, and the source electrode is connected to acommon electric potential line 9 extending in the same direction as thedirection of the data line, and the drain electrode is connected to oneelectrode (cathode) of the organic light-emitting diode 24. The otherelectrode (anode) of the organic light-emitting diode 24 is connected toa power supply line common to all pixels and is kept at a constantelectric potential Va. The organic light-emitting diodes 24 eachemitting any of colors red, green and blue are placed in a matrix formin a prescribed order.

According to the configuration described above, when the switchingtransistor 21 is in an on state by the scan signal, an image signal fromthe data line is written in the storage capacitor 23 via the switchingtransistor 21. Consequently, the gate electrode of the drivingtransistor 22 is kept at an electric potential corresponding to theimage signal by the storage capacitor 23 even if the switchingtransistor 21 is in an off state. The driving transistor 22 is kept at adriving state of a source-ground mode excelling in constant currentproperty, and the current is kept running through the organiclight-emitting diode 24 to maintain the light-emitting state. At thistime, the light emitting luminance depends upon the data written in thestorage capacitor 23. The stopping of the light emission is carried outby turning the driving transistor 22 off.

Subsequently, a configuration of an embodiment of the OLED displaydevice according to the present invention will now be described byreferring to FIG. 10 and FIG. 11. FIG. 10 is a partially cross-sectionalview showing a schematic configuration for explaining the basicconfiguration the OLED display device according to the presentinvention. FIG. 11 is a partially cross-sectional view showing aschematic configuration for explaining the basic configuration the OLEDdisplay device according to the present invention, which displays fullcolor images. In FIG. 11, the organic layer 110 is configured to bepatterned light emitting organic layers for each primary colors, i.e., ared light emitting organic layer 110R, a green light emitting organiclayer 110G, and a blue light emitting organic layer 110B. This displaydevice is an OLED display device having a so-called top-emittingstructure, in which lights are emitted from the direction reverse to thesubstrate on which the organic light emitting diode is formed.Hereinafter, the OLED display device is sometimes abbreviated as the“display device”.

In FIG. 10, the OLED display device according to this embodiment has aflat first substrate 6 made of a glass or such on which a silicon filmin an island state is placed for forming a switching transistor 21 shownin FIG. 9 (not shown), a driving transistor 22, and a gate insulationlayer formed thereon. On the gate insulation layer, a gate electrode,gate lines, an electrode for storage capacitor are formed, andthereafter, a source and drain ranges are formed on the gate electrodein a self alignment manner. Furthermore, a first interlayer insulationlayer 50 is provided, and data lines r common electric potential line,and an electrode for a storage capacitor are formed via a contact hole.Furthermore, a flat layer 52 comprising a second interlayer insulationlayer 51 and an insulation material is stacked, on which a reflectiveelectrode 300 serving as a cathode for the organic light-emitting diode24 is formed in an island form. The reflective electrode 300 isconnected to the drain of the driving transistor 22 via the contact hole53 of the second interlayer insulation layer 51 and the flat layer 52.

On the flat layer 52, a dividing wall 60 is formed so as to surround thearea where the reflective electrode 300 is formed. In this case, thedividing wall 60 may covered with a part of the area of the reflectiveelectrode 300 such as the contact hole. It is desirable for the dividingwall 60 to at least select a material which has no or little reflectionof the light corresponding to the wavelength range at which thepolarization separator is reflected. Specifically, the dividing wallpreferably serves as means for preventing the reflection of the lighthaving a wavelength corresponding to the wavelength range at which thepolarization separator is reflected. For example, many of photoresistresins which can form a pattern by a photolithographic process in whicha light having a short wavelength such as ultraviolet light or nearultraviolet light generally absorbs a light having a short wavelengthrange corresponding to blue light, these material can be used as thematerial for the dividing wall. Also, photosensitive resin materialhaving a light-absorbing pigment or dye dispersed therein may be used asthe material for the dividing wall. The material for the dividing wall60 may be formed by a photolithographic process.

The organic layer 110 which has emissive layers, each of which emits anyof red, green and blue colors are patterned on the reflective electrode300 in a prescribed position The organic layer 110 may be selected fromthe configurations and materials described above. The color patterningof the organic layer 110 can be carried out by the conventionally knownselective deposition method of vacuum-evaporating an organic filmutilizing a shadow mask in the case were the organic layer comprises alow molecular material (for example, see S. Miyaguchi, et., al,:“Organic LED Fullcolor Passive-matrix Display”, Journal of the SID, 7,3, pp 221-226 (1999). In this process, the dividing wall 60 may be usedas a stopper element for the shadow mask.

Also, in the case where the organic layer 110 comprises a polymericmaterial, the conventionally known ink-ject patterning technique can beused (for example, see T. Shimoda, et., al.; “Multicolor PixelPatterning of Light-Emitting Polymers by Ink-Jet Printing”, SID 99DIGEST, 376 (1999). In this process, the dividing wall 60 may be actedas a bank for separating the pixel ranges.

A transparent electrode 200 serving as an anode is formed on the entiresurface of the organic layer 110 as the opposite electrode. Optionally,a protective layer 70 comprising a transparent insulating material isformed on the transparent electrode 200. The formation of the protectivelayer 70 is for the purpose of protecting the transparent electrode 200and for making it easy to deposit members to be placed thereon. As theprotective layer 70, those which are made of transparent organicmaterials such as acrylic resins, benzo cyclobutadiene resins, polyimideresins. These organic materials can relatively easily be planarized byfilm-formation through a spin coater.

A second substrate 90, which comprises an optically isotropic,transparent, and flat substrate is placed on the protective layer. Onone surface of the second substrate 90 is formed a polarizationseparator 500 and on the other surface thereof are stacked a phase plate700 and a polarizer plate 600. The second substrate 90 is stacked sothat the surface where the polarization separator 500 is faced to thesurface of the first substrate 6 where the organic layer 110 is formed.As the materials for the second substrate, a transparent glass, apolymer film such as polycarbonate film, and triacetyl cellulose film,formed by a casting method; an optically isotropic plastic film or sheetsuch as alicyclic acryl resin formed by an injection molding (OPTOREZ®produced by Hitachi Chemical Co., Ltd.).

In the case where the polymer film or the resin sheet is used, it isimportant for enlarging a lifetime of the organic layer to be impartedto a gas barrier property, e.g., by subjecting a gas barrier treatment(such as the formation of a gas barrier layer) or by placing a glasshaving a thickness of several ten microns. If it is possible to besubjected to a treatment that sufficient gas barrier property can beobtained, the second substrate may be omitted to construct a stackcomprising the polarization separator 500, the phase plate 700 and thepolarizer plate 600. As described above, the polarization separator 500comprising the cholesteric liquid crystal layer having a main wavelengthrange of the selective reflection corresponding to the blue light isused.

As a process for forming the polarization separator 500 comprising thecholesteric liquid crystal layer on the second substrate 90, a processcan be mentioned, which comprises applying a liquid crystal polymer onthe oriented second substrate 90, adjusting the temperature to aprescribed temperature utilizing the thermochromic property of theselective reflection wavelength, fixing the structure through aphotopolymerization to form a cholesteric liquid crystal layer having adesired selective reflection wavelength, but the present invention isnot restricted thereto.

Also, the cholesteric liquid crystal layer having a desired selectivereflection wavelength having been formed on a triacetyl cellulose filmmay be adhered on the second substrate 90 by a transparent adhesive.Optionally, a transparent protective layer may be provided on thecholesteric liquid crystal layer.

The phase plate 700 and the polarizer plate 600 are stacked on thesurface reverse to the surface having the polarization separator 500formed thereon. The phase plate 700 and the polarizer plate 600 are asdescribed above, and they are adhered by an acrylic transparentadhesive, respectively. No second substrate may be used and thepolarization separator may be directly formed on the phase plate. Inthis case, a material, which never changes characteristics such as thephase difference of the phase plate in the process for forming thecholesteric liquid crystal layer may preferably used.

The full surface of the first substrate 6 and the full surface of thesecond substrate 90 may be brought into closely contact with each otherso that no gas is incorporated. In terms of the reason which will bedescribed later on, however, it is preferable to apply a sealing agenthaving a spacer material such as beads and a rod incorporated therein tothe circumference of the display portion in a frame state to seal andadhere them in the state where nitrogen is sealed in a space 80.

Subsequently, the display operation of the OLED display device 1according to this embodiment will now be described by referring to FIG.9, FIG. 12 and FIG. 13 each is an explanatory drawing showing oneembodiment of the display operation of the OLED display device accordingto the present invention, where FIG. 12 is a time chart of the voltagesVG1, VG1, . . . VGm gradually applied to the gate lines G1, G2, . . .Gm, and FIG. 13 is a time chart which exemplifies the voltage situationsof the gate voltage VG1 and the data voltage VD1 positioned at firstline and first column, and the storage capacitor 23.

As shown in FIG. 12, voltages VG1, VG1, . . . VGm, which gradually turnthe switching transistor 21 on, are applied to the gate lines G1, G2, .. . Gm. At the time t=t₀, when the voltage VG1, which turns theswitching transistor 21 on, is applied to the gate line G1, one scanningin the vertical direction is completed within one frame period T1, andthe turning on voltage is applied to the gate G1 at the time t=t₀÷Tf. Inthis driving scheme, the period for applying the turning on voltage toone gate line is not more than Tf/m. Generally, the Tf value which isused is approximately 1/60 second.

When the turning on voltage is applied to a given gate voltage, all ofthe switching transistors connected to that gate lines are the on state,and being synchronized therewith, the data voltages corresponding to theimage signal are applied to the data lines D1, D2, . . . Dn. Such manneris called line-gradul scanning manner, and is a manner generally used inan active matrix liquid crystal.

Subsequently, paying attention to the pixel positioned at first line andfirst column, the voltage states of the gate voltage VG1 and the datavoltage Vd and the storage capacitor 23 will be described by referringto FIG. 13. At the time t=t₀, the value of the data voltage VDsynchronized with the voltage VG1 is taken as d1, and the data voltageat the next frame t=t₀+Tf is taken as d2. In this case, while theturning on voltage is applied to the gate line G1, these data voltagesare stored in the storage capacitor 23, and during the course of 1frame, these data voltages are kept at these values. These voltagevalues define the gate voltage of the driving transistor 22 and thecurrent value running through the transistor is controlled and, thus, aconstant current defined by the voltage (constant) applied by them andthe common electric potential line 9 and the voltage Va (constant) runsthrough the organic light-emitting diode to bring about a prescribedlight emission.

Specifically, being synchronized with the application of the turning onvoltage to the gate line corresponding to the pixel which should controlthe light emission, the voltage corresponding to the image informationis applied via the data line, whereby the light emission of the pixelcan be controlled. Consequently, the light emission of a plurality ofthe pixels making up the display portion is controlled depending uponthe image information, whereby a desired image can be displayed. Sincethe response time from the application of the voltage between both endsof the cathode and the anode of the organic light-emitting diode to thestarting of the light emission is usually not more than 1 microsecond,the image displaying, which can follow up rapidly moving image can berealized.

Here, when the current running through the organic light-emitting diodeis increased, the amount of the light emission of the organiclight-emitting diode becomes large to obtain bright displaying as arule, but the power consumption is increased in so much, the lifetime ofthe pixel (for example, the period until the luminance becomes half theinitial luminance) is decreased.

As described above, the OLED display device 1 according to thisembodiment can effectively utilize the light corresponding to the bluelight, which has conventionally been absorbed on the polarizer plate tobe lost, by the action of the polarization separator and, thus, theluminance can be improved, and the power consumed by the organiclight-emitting diode when a white color is displayed can be decreased.For this reason, a display device which has a high luminance and candisplay a bright image using the same power consumption can be realized.Alternatively, when the luminance (brightness) is the same, the currentrunning through the organic light-emitting diode can be decreased and,thus, the power consumption can be decreased and, what is more, thedisplay device having a long lifetime can be realized.

Furthermore, as described above, by the action of the polarizationseparator, the OLED display device 1 according to this embodiment has anadvantage that the excitation purity of the light practically emittingto the side of the viewer is improved by the light emission itselfemitted from the emissive layer with regard to the blue light.

In the OLED display device 1 according to this embodiment, the dividingwall 60 as shown in FIG. 10 is provided around the light-emitting rangeof the organic light-emitting diode constituting each pixel. Thedividing wall 60 does not reflect at least the light having thewavelength of the reflection at the polarization separator 500. In thiscase, as for the light entering in the light emission range of theorganic light-emitting diode amongst the ambient light entering in theOLED display device from the outer circumference under a brightenvironment, the light having a wavelength corresponding to thewavelength range reflected at the polarization separator 500 isreflected, but as for the light entering in the dividing wall, the lighthaving a wavelength corresponding to the wavelength range reflected atthe polarization separator 500 is not reflected, and even if the lightshaving a wavelength other than the wavelength range reflected at thepolarization separator 500 are reflected, they are not emitted out ofthe display device because they are absorbed on the polarizer plate.Consequently, the reflection of the ambient light so much as the rangeof the dividing wall and, thus, the contrast ratio under a brightcondition is enhanced.

Furthermore, since the dividing wall prevents the light emitted from theemissive layer and reflected at the polarization separator from beingleaked into another pixel, it has an effect for preventing cross-talk orblooming. Specifically, since each pixel is optically separated by thedividing wall, high quality display without cross-talk or blooming canbe obtained.

The dividing wall 60 can be acted as a spacer at the time of depositingthe first substrate having the organic light-emitting diode formedthereon on the second substrate having the polarization separator formedthereon. In this case, it has an effect for preventing a defect due thecontact of the organic light-emitting diode with the polarizationseparator.

Furthermore, the polarization separator, the phase plate, and thepolarizer plate are formed in a plane form, and there is no requirementfor the alignment with the pixel whose organic layer is patterned and,thus, the effect for improving the productivity can be obtained. Here,an embodiment has been described in which the polarization separator andthe organic light-emitting diode are formed on the different substrates,and they are finally deposited. This is because in the case of formingboth parts on the same substrate, for example, forming the polarizationseparator on the substrate having the organic layer and the like alreadybeing formed, there is a possibility to bring about deficiency such asthe deterioration of the organic layer, at the time of forming thecholesteric liquid crystal layer making up the polarization separator.Specifically, when the polarization separator and the organiclight-emitting diode are formed on the different substratesrespectively, the degree of the freedom in each state is increased, andthey are not deteriorated with each other, making it possible toconstruct the device having much more high performance. However, forexample, if a highly resistant organic material is developed in the nearfuture, the polarization separator and the organic light-emitting diodemay be formed on the same substrate.

In the OLED display device according to the present invention, if thedistance between the polarization separator and the reflective electrodeis long, there would be possibility to bring about trouble, i.e., thelight reflected at the polarization separator is leaked into the pixelother than the corresponding pixel, leading to the decreasing of theresolution, the light emitted from the emissive layer or the lightreflected at the polarization separator are absorbed on the dividingwall, decreasing the light directing toward the viewer. For this reason,the distance between the polarization separator and the reflectiveelectrode, which is as short as possible, is preferable in terms of theimage quality and the efficiency for utilizing the emitted light.

In the case where a substrate is intervened between the organiclight-emitting diode and the polarization separator, if the substrate ismade of glass, the thickness of the substrate becomes several hundredsmicrons, or even if the substrate is made of a plastic film, thethickness becomes not less than several ten microns, leading to a longdistance between the polarization separator and the reflectiveelectrode. In contrast, the display device according this embodiment isconfigured that the light from the organic light-emitting is emittedfrom the reverse direction to the first substrate having the organiclight-emitting diode formed thereon, and the polarization separator isstacked via the transparent, thin plate layer or insulation layer. Thisconfiguration makes it possible to decrease the distance between thepolarization separator and the reflective electrode to be not more than10 microns and, thus, the light absorbed on the dividing wall or such tobe lost can be reduced to improve the efficiency for utilizing the lightemitted from the emissive layer, obtaining much more bright display. Inthis case, since the light reflected at the polarization separator isnever leaked into the reflective electrode of the different pixel todecrease the resolution or bring about blooming, the effect forobtaining high quality display can be obtained.

In the OLED display device according this embodiment, in the case wherea space sealed with a gas is provided between the polarization separator500 and the protective layer 70 formed on the transparent electrode 200,it is desirable that the sum thickness of the transparent electrode 200and the protective layer 70 is set to be not more than quarter thewavelength of the light emitted from the emissive layer. Here, in thecase where there is a layer having a thickness longer than thewavelength of the light emitted from the emissive layer, and areflectance higher than that of nitrogen or oxygen, on the top of thetransparent electrode of the organic emissive layer, a part of the lightemitted from the emissive layer is wave-guided to the direction parallelto the planes of the first and second substrates with repeating thetotal reflection at the interface between the layer having a highreflectance and a layer having a low reflectance such as air, decreasingthe light emitted to the side of the viewer.

In contrast, in the case where the sum thickness of the transparentelectrode 200 and the protective layer 70 is set to be not more thanquarter the wavelength of the light emitted from the emissive layer andthe space sealed with a gas is provided between the polarizationseparator 500 and the protective layer 70 formed on the transparentelectrode 200, the light emitted from the emissive layer is passedthrough the organic layer, the transparent electrode and the protectivelayer with little wave-guiding towards the direction parallel to thesubstrate, and then is emitted to the space 80. The light entering inthe space 80, then enters in the polarization separator 500, and ispassed through the polarization separator 500 and the second substrate80 without repeating the total reflection toward the direction parallelto the substrates, and is then emitted to the side of the viewer. Forthis reason, much bright display can be obtained.

The arrangement of the pixels constructing the display portion of thisembodiment may be any arrangement such as a stripe arrangement, a mosaicarrangement, a delta arrangement and the like, and the arrangement maybe suitably selected to meet the specification of the display device.Also, whereas the display device which drives an active matrix has beendescribed in the embodiment, the present invention is not restrictedthereto. Specifically, a passive matrix driving may be applied to thedisplay device according to the present invention in which no switchingdevice such as TFT is provided, and the electrodes for light-emittingdevices of the present invention are directly connected to verticalscanning lines and horizontal scanning lines to be driven.

Subsequently, another embodiment of the present invention will bedescribed. FIG. 14 is a partially cross-sectional view showing aschematic configuration of another embodiment of the OLED display deviceaccording to the present invention. This display device has abottom-emitting structure in which the light is emitted from thesubstrate on which the organic light-emitting diode is formed. Thisdisplay device is configured so that in the display device having atop-emitting structure having been described by referring to FIG. 10 orsuch, the organic light-emitting diode 24 composed of the transparentelectrode 200, the organic layer 110, and the reflective electrode 300is formed upside-down. Different from the embodiment of the top-emittingstructure described above, in this embodiment, the electrode of theorganic light-emitting diode 24 connected to the driving transistor 22is the transparent electrode 200. For this reason, the construction ofthe circuit is sometimes changed, but since each pixel comprises theorganic light-emitting diode 24, the storage capacitor (not shown), theswitching transistor, and the driving transistor 22 as in thetop-emitting structure, and since the basic operation is substantiallythe same as that of the top-emitting structure, parts having similarfunctions are referred to the same number and the description thereofwill be omitted.

In this embodiment, the light is emitted from the side of the firstsubstrate 6 comprising a transparent material such as glass on which theorganic light-emitting diode 24 is formed. For this reason, opposite thesurface on which the organic light-emitting diode 24 is formed, thepolarization separator 500, the phase plate 700, and the polarizer plate600 are placed and stacked on the first substrate 6 in this order. Thesurface of the first substrate 6 having the organic emissive layerformed thereon is sealed with a sealing plate 800 comprising glass,stainless steel or a resin having been subjected to gas-barriertreatment not so as to be contact with the open air. The first substrate6 and the sealing plate 800 are sealed and adhered by applying a sealingagent having a spacer material such as beads and a rod incorporatedtherein to the circumference of the display portion in a frame state,and sealing nitrogen into the space 80, and optionally incorporating adesiccant.

FIG. 15 is a partial plane view schematically showing the configurationof the pixel portion 20 of the OLED display device according to anotherembodiment of the present invention viewing from the first substrate 6.In the case of the OLED display device having a bottom-emittingstructure as in this embodiment, with regard to the light emission range24E of the organic light-emitting diode 24, the range is shared with thestorage capacitor 23, switching devices such as the thin film transistorTFT, and the lines and, thus, a wide light emission range like thetop-emitting structure cannot be secured as in the case of thetop-emitting structure.

For this reason, when the ranges other than the light emission range 24Eare configured to be no or little reflection of the light correspondingto the wavelength range reflected by the polarization separator 500, thereflection of the ambient light can be markedly decreased. Consequently,it is preferable if an anti-reflective coating (not shown) is formed onthe portions other than the light emission range of the organiclight-emitting diode on the first substrate. The anti-reflective filmmay be a film, which is free of or reduced reflection of the lightcorresponding to the wavelength range reflected by the polarizationseparator 500. In this case, the film itself or a dye or pigmentcontained therein may absorb the corresponding light. Alternatively, theanti-reflective film may realize no or little reflection of the lightcorresponding to the wavelength range reflected by the polarizationseparator 500 by the interference effect of the light due to transparentor translucent films each having different reflectance. Furthermore, nonew film is added, and the storage capacitor 23, which occupies arelatively large range in the pixel portion 20, may be functioned as theanti-reflective film.

FIG. 16 is a partial cross-sectional view showing one example of theconfiguration of the storage capacitor 23 in another embodiment of theOLED display device according to the present invention. The storagecapacitor 23 is composed of the stack, from the side of the firstsubstrate 6, polysilicon (poly-Si) 23C, silicon oxide (SiO₂) 23B, andtitanium-tungsten (Ti—W) 23A in this order, where the thickness ofpolysilicon film is 50 nm, that of silicone oxide film is 100 nm, andthat of titanium-tungsten film is 150 nm. Although not being furtherdepicted, the storage capacitor 23 is composed of the stack of SiO₂ andAl, but such a configuration is omitted herein.

In this case, amongst the ambient light entering in the OLED displaydevice from the outside under a bright environment, with regard to thelight entering in the light emission range 24 of the organiclight-emitting diode, the light corresponding to the blue light whichcorresponds to the wavelength range of the reflection at thepolarization separator 500 is reflected, but with regard to the lightentering in the storage capacitor 23, the reflection of the lightcorresponding to the blue light which corresponds to the wavelengthrange of the reflection at the polarization separator 500 is decreased,the lights other than wavelength range of the reflection at thepolarization separator 500 are not emitted out, because even if they arereflected at any portions other than the light emission range 24, theyare absorbed on the polarizer plate. Consequently, the reflection of theambient light is decreased so much as the storage capacitor 23, thecontrast ratio under bright environment can be enhanced.

In order to decrease the reflect the light passed between the lines andthe switching devices, the dividing wall is configured that thereflection of the light corresponding to the wavelength range reflectedby the polarization separator is eliminated or reduced, even in the caseof the bottom-emitting structure, the reflection of the ambient lightcan be remarkably decreased.

Furthermore, amongst the light emission range, in the light emissionrange, which emits a light having a wavelength different from the mainwavelength range reflected by the polarization separator, i.e., in thisembodiment, the light emission range, which emits red and green lightsother than the blue light, the reflection of the light having the mainwavelength range reflected by the polarization separator is preferablydecreased. Specifically, the thickness of each layers making up theorganic light-emitting diode is controlled to be a condition where thereflection of the light having the main wavelength range reflected bythe polarization separator is decreased by the interference effect.Alternatively, color filers corresponding to respective colors, i.e., afiler which transmits the red color and absorbs the blue colors at thelight emission side of the red light emission range, and a filer whichtransmits the blue color and absorbs the blue colors at the lightemission side of the blue light emission range, are preferably provided.In this case, the reflection of the ambient light can be remarkablydecreased without loosing the lights having desired colors.

Subsequently, another embodiment of the present invention will bedescribed. FIG. 17 is a partial cross-sectional view showing the basicconfiguration of the OLED display device, which displays full colors,according to another embodiment of the present invention. This displaydevice has the same basic configuration as that of the top-emittingstructure having been described in the embodiment referring to FIG. 10,and FIG. 11, except that the cholesteric liquid crystal layer 500 makingup the polarization separator 500 is selectively placed on the organiclayer 110B for the blue light emission. The same parts as those in theaforementioned embodiment are referred to the same number, and thedescription thereof will be omitted.

As shown in FIG. 17, the display device according to the presentinvention is configured so that the cholesteric liquid crystal layer 500making up the polarization separator 500 is selectively placed on theorganic layer 110B for the blue light emission On the surface of thesecond substrate where the cholesteric liquid crystal layer 500, aplanarized plate 510 may be provided in order to eliminate the stepformed through the selectively formed cholesteric liquid crystal layer.As the planarized layer 510, organic materials such as acrylic resins,benzo cyclobutadiene resins, polyimide resins may be utilized. Thesurface of the organic material can relatively easily be planarized byfilm-formation through a spin coater.

In this embodiment, similar to the embodiment described previously, bythe action of the polarization separator, the light corresponding to theblue light, which has conventionally been absorbed on the polarizerplate to be lost can effectively utilized and, thus, the luminance ofthe single blue color can be improved, and the power consumed by theorganic light-emitting diode when a white color is displayed can bedecreased. Furthermore, by the action of the polarization separator, theOLED display device 1 according to this embodiment has an advantage thatthe excitation purity of the light practically emitting to the side ofthe viewer is improved by the light emission itself emitted from theemissive layer with regard to the blue light. For this reason, thedisplay range of the display device is advantageously widened.Furthermore, in this embodiment, since the polarization separator is notprovided on any ranges other than the light emission range of theorganic light-emitting diode for the blue light, the reflection of theambient light decreased one third or less, improving the contrast ratiounder a bright condition.

Subsequently, still another embodiment of the present invention will bedescribed. FIG. 18 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to another embodimentof the present invention. This display device has the same basicconfiguration as that of the top-emitting structure having beendescribed in the embodiment referring to FIG. 10, and FIG. 11, exceptthat a plurality of the cholesteric liquid crystal layers are stacked tomake up the polarization separator 500. The same parts as those in theaforementioned embodiment are referred to the same number, and thedescription thereof will be omitted.

The polarization separator 500 is composed of a stack comprising acholesteric liquid crystal layer 500B having a wavelength range of themain selective reflection at the wavelength corresponding to the blueand conditioned to obtain the maximum reflectance, a cholesteric liquidcrystal layer 500G having a wavelength range of the main selectivereflection at the wavelength corresponding to the green and conditionednot to obtain the maximum reflectance, and a cholesteric liquid crystallayer 500R having a wavelength range of the main selective reflection atthe wavelength corresponding to the red and conditioned not to obtainthe maximum reflectance. Specifically, the polarization separator 500 inthe display device of this embodiment have a reflectance of the lightcorresponding to the blue, and a decreased reflectance of the lightsother than blue, especially, green, which has a high relative luminousefficiency in a photopic vision.

The selective reflection of the above-mentioned cholesteric liquidcrystal layer depends upon a number of helical pitches. Consequently,the number of the helical pitches in the cholesteric liquid crystallayer 500G having a wavelength range of the main selective reflection atthe wavelength corresponding to the green, and in the cholesteric liquidcrystal layer 500R having a wavelength range of the main selectivereflection at the wavelength corresponding to the red are set to be lessthan 20 pitches, preferably not more than 10 pitches, to decrease thereflectance of the selective reflection. The number of the pitches canbe decreased by thinning the thickness of the cholesteric liquid crystallayer.

FIG. 19 is a drawing showing one example of a spectral transmittance ofthe cholesteric liquid crystal layer making up the polarizationseparator in still another embodiment of the present invention, and is agraph showing the wavelength-dependency of the transmittance when anun-polarized light enters in the cholesteric liquid crystal layer. InFIG. 19, the wavelength range having a low transmittance corresponds tothe wavelength range of the selective reflection. In this embodiment,the light, which is absorbed on the polarizer plate, is decreased evenat the wavelength having a high relative luminous efficiency and, thus,the bright display can be advantageously obtained. On the other hand,the reflection of the ambient light is somewhat increased due to thecholesteric liquid crystal layer 500G having a wavelength range of themain selective reflection at the wavelength corresponding to the greenand the cholesteric liquid crystal layer 500R having a wavelength rangeof the main selective reflection at the wavelength corresponding to thered. However, since the reflectance of the selective reflection at thecholesteric liquid crystal layer is suppressed to be low, the reflectionof the ambient light is suppressed in so much.

What is important here is that the reflection of green, which has a highrelative luminous efficiency in a photopic vision, should be smallerthan the reflection of blue. This decreases the reflection of theambient light, enhancing the contrast ratio under a bright condition. Inthis embodiment, it is possible that the wavelength range of theselective reflection possessed by the cholesteric liquid crystal layerhaving a wavelength range of the main selective reflection at thewavelength corresponding to the red or blue is set to be wide to therebyobtain substantially similar selective reflection at the visiblewavelength range except for the wavelength range corresponding to theblue, so that the color change viewing from a diagonal angel due to theangle-dependency of the selective reflection of the cholesteric liquidcrystal layer may be suppressed. Also, in the display device of thisembodiment, the order of the lamination of the cholesteric liquidcrystal layer is not restricted to the order described in the figure.

Instead of the stack of a plurality of the cholesteric liquid crystallayers each having a different helical pitch, the cholesteric liquidcrystal layer whose helical pitch is continuously changed may also beused. In this case, when the wavelength range obtaining the maximumreflectance of the cholesteric liquid crystal layer is set to be withinthe wavelength range corresponding to blue, which is of a low relativeluminous efficiency in a photopic vision, specifically not less than 510nm, more desirably not less than 490 nm, a high contrast ratio can beobtained under a bright condition.

Subsequently, still another embodiment of the present invention will bedescribed. FIG. 20 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to still anotherembodiment of the present invention. This display device has the samebasic configuration as that of the top-emitting structure having beendescribed in the embodiment referring to FIG. 10, and FIG. 11, exceptthat the cholesteric liquid crystal layer having a plurality ofpatterned ranges is used as the polarization separator 500. The sameparts as those in the aforementioned embodiment are referred to the samenumber, and the description thereof will be omitted.

The polarization separator 500 of this embodiment is configured so thata cholesteric liquid crystal layer 500B having a wavelength range of themain selective reflection at the wavelength corresponding to the blueand conditioned to obtain the maximum reflectance is placed on theorganic layer 110B for the blue light emission, a cholesteric liquidcrystal layer 500G having a wavelength range of the main selectivereflection at the wavelength corresponding to the green and conditionednot to obtain the maximum reflectance is placed on the organic layer110G for the green light emission, and a cholesteric liquid crystallayer 500R having a wavelength range of the main selective reflection atthe wavelength corresponding to the red and conditioned not to obtainthe maximum reflectance is placed on the organic layer 110R for the redlight emission.

Specifically, the polarization separator 500 in the display device ofthis embodiment is composed of the cholesteric liquid crystal layerswhich are patterned to correspond to the light emission layer making upthe pixel portion, and amongst the patterned cholesteric liquid crystallayers, the reflectance of the cholesteric liquid crystal layer having awavelength range of the main selective reflection at the wavelengthcorresponding to the blue is set to be high, and the reflectance of thecholesteric liquid crystal layer having a wavelength range of the mainselective reflection at the wavelength corresponding to the green, whichhas a low relative luminous efficiency in a photopic vision is set to below. It is preferable to form a black matrix 520 between the patternedcholesteric liquid crystal layers. The black matrix 520, which can beused, includes, but are not restricted to, a black matrix comprising aphotoresist resin having chromium, chromium oxide, or photo-absorbingpigment dispersed therein. In this case, in order to take a large marginfor aligning the patterned cholesteric liquid crystal layer and thepixel, the opening of the black matrix is desirably larger than thelight emission range.

As described above, since the selective reflection of theabove-mentioned cholesteric liquid crystal layer depends upon a numberof helical pitches, the number of the helical pitches in the cholestericliquid crystal layer 500G having a wavelength range of the mainselective reflection at the wavelength corresponding to the green, andin the cholesteric liquid crystal layer 500R having a wavelength rangeof the main selective reflection at the wavelength corresponding to thered are set to be less than 20 pitches, preferably not more than 10pitches, to decrease the reflectance of the selective reflection. Thenumber of the pitches can be decreased by thinning the thickness of thecholesteric liquid crystal layer.

In this embodiment, since the light absorbed on the polarizer plate evenat a wavelength having a high relative luminous efficiency, brightdisplay can advantageously be obtained. On the other hand, thereflection of the ambient light is somewhat increased due to thecholesteric liquid crystal layer 500G having a wavelength range of themain selective reflection at the wavelength corresponding to the greenand the cholesteric liquid crystal layer 500R having a wavelength rangeof the main selective reflection at the wavelength corresponding to thered. However, since the reflectance of the selective reflection at thecholesteric liquid crystal layer is suppressed to be low, the reflectionof the ambient light is suppressed in so much. Since the cholestericliquid crystal layer is patterned in this embodiment, the reflectionincreased due to the cholesteric liquid crystal layer for each color isrestricted to the patterned range, and the reflection area is one thirdor less, the reflection of the ambient light can be further suppressed.

Here, what is important is that the reflection of green, which has ahigh relative luminous efficiency in a photopic vision, should besmaller than the reflection of blue. This decreases the reflection ofthe ambient light, enhancing the contrast ratio under a brightcondition.

Subsequently, still another embodiment of the present invention will bedescribed. FIG. 21 is a partial cross-sectional view showing the basicconfiguration of the OLED display device according to still anotherembodiment of the present invention. This display device has the samebasic configuration as that of the top-emitting structure having beendescribed in the embodiment referring to FIG. 10, and FIG. 11, exceptthat a polarization separator (hereinafter referred to as the “linearpolarization separator) 550, which reflects a linearly polarized lightcomponent having a prescribed wavelength range, and transmits othercomponents is used as the polarization separator, and the position ofthe phase plate is changed. The same parts as those in theaforementioned embodiment are referred to the same number, and thedescription thereof will be omitted.

As shown in FIG. 21, the display device of this embodiment is configuredthat from the side of the transparent electrode 200, the phase plate700, the linear polarization separator 550, and the polarizer plate 600are placed. The linear polarization separator 550 has a function that alinearly polarized light component having a prescribed wavelength rangeis reflected and a light component having a wavelength rangeperpendicular to the former is transmitted. Various configurations ofthe linear polarization separator 550 may be considered. For example, abirefringent reflective polarizer film comprising different birefringentlayers alternatively stacked as described in WO95/27919, which isincorporated herein by reference, and one produced by piling two prismallays whose top angle is 90 degree, and forming a polarizationseparating surface due to the dielectric multilayers on the stackedportion as described in SID 92 Digest (p 427) can be used.

The central wavelength of the reflection at the linear polarizationseparator is preferably from 400 nm to 490 nm, and more preferably from420 nm to 480 nm, and the wavelength range of the reflection isdesirably not more than 510 nm. This is for the purpose of minimizingthe reflection of the ambient light, and for effectively utilizing aneffective light as a blue light to increase the color purity of the bluewhereby the total efficiency of the display device is improved, similarto the embodiment described previously.

The phase plate 700 and the polarizer plate 600, which can be used arethose which make up the circular polarizer plate in the prior art.Specifically, the polarizer plate 600 transmits a specific linearlypolarized light amongst the lights passing there-through, and absorbs alinearly polarized light having a perpendicular to the former. The phaseplate 700 is made up of the material serving as a quarter wave platewhich converts the linearly polarized light passing through thepolarizer plate 600 into a substantially circularly polarized light.While the linear polarization separator 550 is placed between thepolarizer plate 600 and the phase plate 700 making up the circularpolarizer plate, at this time, the linear polarization separator 550 isplaced in such a manner that the transmitting axis of the linearlypolarized light of the linear polarization separator 550 is accordedwith the transmitting axis of the linearly polarized light of thepolarizer plate 600.

Subsequently, the operation of the display device according to thisembodiment will now be described. When a direct current voltage isapplied between the transparent electrode 200 and the reflectiveelectrode 300, a light with a prescribed wavelength is emitted from theemissive layer making up the organic layer. Amongst the light emittedfrom the emissive layer, the light directing towards the reflectiveelectrode 200 is transmitted through the transparent electrode 200 andthrough the phase plate 700, and then enters in the linear polarizationseparator 550. On the other hand, amongst the light emitted from theemissive layer, the light directing towards the reflective electrode 300is reflected at the reflective electrode 300, and then similarlytransmitted through the transparent electrode 200 and through the phaseplate 700, and then enters in the linear polarization separator 550. Atthis time, since the light emitted from the emissive layer and enteringin the linear polarization separator 550 is an un-polarized light, thelinearly polarized light component which is a light corresponding to theblue and which should be absorbed on the polarizer plate, is reflectedand the light components other than the former are transmitted. Amongstthe light transmitted through the liner polarization separator 550, thelight corresponding to the wavelength range of the reflection at thelinear polarization separator 550 is transmitted through the polarizerplate 600 and is directed towards the viewer 10000, but the lightshaving a wavelength other the former are absorbed half on the polarizerplate 600 and then directed towards the viewer 10000.

On the other hand, the light reflected at the linear polarizationseparator 550 is passed through the phase plate and is directed towardsthe reflective electrode 300. At the time of being passed through thephase plate 700, the light has an influence of the phase plate 700 to bea circularly polarized light. At the time of the reflection at thereflective electrode 300, the light directing towards the reflectiveelectrode 300 becomes a circularly polarized light whose helicitydirection is reverse, and at the time of being passed through the phaseplate 700 again, the light has an influence of the phase plate 700 to beconverted into a linearly polarized light, which is transmitted throughthe linear polarization separator 550. For this reason, it istransmitted through the linear polarization separator 550 and thepolarizer plate 600 to be directed towards the viewer 10000.

Specifically, in the display device of this embodiment, since the lighthaving a wavelength corresponding to the blue, amongst the light emittedfrom the light-emitting layer, is directed towards the viewer 10000 withlittle absorption on the polarizer plate, the luminance of the singlecolor of the blue is enhanced, and the power consumed at the time ofdisplaying white color is decreased as is the embodiment describedpreviously. For this reason, a display device which has a high luminanceand can display a bright image using the same power consumption can berealized. Alternatively, when the luminance (brightness) is the same,the current running through the organic light-emitting diode can bedecreased and, thus, the power consumption can be decreased and, what ismore, the display device having a long lifetime can be realized.

Subsequently, the ambient light, which enters in the display device fromthe circumferences under bright conditions will now be described. Theambient light entering in the display device from the circumferences isgenerally un-polarized. Amongst the ambient light, when being passedthrough the polarizer plate 600, a prescribed linearly polarized lightis absorbed, and the linearly polarized light perpendicular thereto isreflected. The linearly polarized light having been transmitted throughpolarizer plate 600 is also transmitted through the linear polarizationseparator 550 and, by the action of the phase plate 700 to be acircularly polarized light. The light having been passed through thephase plate 700 becomes a circularly polarized light having a reversehelicity direction as a result at the time of being reflected at thereflective electrode 300. The light reflected at the reflectiveelectrode 300 is converted into a linearly polarized light, which isabsorbed at the reflective electrode 300 at this time, at the time ofbeing passed through the phase plate again, and then enters in thelinear polarization separator 550. At the linear polarization separator550, the light having a wavelength corresponding to the blue isreflected, and the lights other than the former are transmitted. Thelights having being transmitted through the linear polarizationseparator 500 is absorbed on the polarizer plate 600, and thus, they arenot returned to the external display device.

On the other hand, the light reflected at the linear polarizationseparator 550 is converted into a circularly polarized light by theaction of the phase plate 700 at the time of being passed through thephase plate 700, and at the time of reflected at the reflectiveelectrode 300 again, it becomes a circularly polarized light having areverse helicity direction. The light reflected at the reflectiveelectrode 300 is converted into a linearly polarized light, which isabsorbed at the reflective electrode 300 at this time, and then passedthrough the linearly polarization separator 550 and polarizer 600 to bedirected towards the viewer 10000.

Specifically, at least half of the ambient light entering in the displaydevice is first absorbed on the polarizer plate 600. The light havingbeen transmitted through the polarizer plate 600 is transmitted throughthe linear polarization separator 550 and the phase plate 700, reflectedat the reflective electrode 300, and again enters in the linearpolarization separator 550. Amongst the lights just mentioned, the lighttransmitted through the linear polarization separator 550 is absorbed onthe polarizer plate 600. Consequently, the light emitted out is just asmall amount of light having a wavelength range reflected at the linearpolarization separator 550. The reflected light is a light having a lowrelative luminous efficiency in a photopic vision, corresponding to theblue, and thus, the luminous reflectance become small. Specifically,similar to the embodiment described previously, since almost all of theambient light is cut even under a bright environment, the black displayis darken, and, thus, the display device of this embodiment has aneffect that display with a high contrast ratio can be realized.

With regard to the full coloration of the OLED display device, severalmanners have been suggested and evidenced. For example, a manner of thecombination of the blue light-emitting element with fluorescent colorchanging mediums (CCM) (referred to as CCM manner), a manner of thecombination of a white light emission with primary color filters of red(R), green (G), and blue (B) (referred to as RGB by white manner) andthe like have been suggested.

In the CCM manner, a fluorescent dye for color changing is excited witha light emitted from a blue emissive layer to convert the blue intogreen and red to obtain emission of primary colors. The RGB by whitemanner is characterized by the simplest production, because the emissivelayer to be produced is only one white emissive layer. When beingapplied to any of the full coloration manners, the OLED display deviceaccording to the present invention can realize display with a highcontrast by placing the polarizer plate, the phase plate and thepolarization separator at the position between the emissive layer andthe viewer.

While the embodiments of present invention have been described, thepresent invention is not restricted to the configurations of theembodiments and various modifications and 6 alternations can be madewithout departing from the technical ideas and sprits of the presentinvention.

This application claims priority from Japanese Patent Application No.2002-181239, the disclosure of which is incorporated herein byreference.

1-20. (canceled)
 21. A display device comprising blue, green, and redlight-emitting devices making up a plurality of pixels placed in amatrix form, wherein the blue, red, and green light-emitting deviceseach comprise: a reflective element, an emissive layer, a phase plate,and a polarizer in this order, and further comprise a polarizationseparator between the emissive layer and the phase plate, whichseparates light into two kinds of polarized light components byreflection and transmission, wherein the polarization separator has areflectance of the wavelength range from 520 nm to 600 nm smaller than areflectance of range not more than 510 nm an emissive layer, and whereinthe emissive layer of at least one of the red and green light- emittingdevices comprises a phosphorescent material.
 22. A light-emitting devicecomprising: a reflective element, an emissive layer, a phase plate, anda polarizer in this order, and further comprise a polarization separatorbetween the emissive layer and the phase plate, which separates lightinto two kinds of polarized light components by reflection antitransmission, wherein the polarization separator has a reflectance ofthe wavelength range from 520 nm to 600 nm smaller than a reflectance ofrange not more than 510 nm an emissive layer, and wherein the emissivelayer emits at least one of green and red light and comprises aphosphorescent material.
 23. A display device comprising blue, green,and red organic light-emitting devices making up a plurality of pixelsplaced in a matrix form, wherein the blue, red, and green organiclight-emitting devices each comprise: a reflective element, an emissivelayer, a phase plate, and a polarizer in this order, and furthercomprise a polarization separator between the emissive layer and thephase plate, which separates light into two kinds of polarized lightcomponents by reflection and transmission, wherein the polarizationseparator has a reflectance of the wavelength range from 520 nm to 600nm smaller than a reflectance of range not more than 510 nm an emissivelayer, and wherein the emissive layer of both of the red and greenorganic light-emitting devices comprises a phosphorescent material. 24.A display device comprising blue, green, and red organic light-emittingdevices making up a plurality of pixels placed in a matrix form, whereinthe blue, red, and green organic light-emitting devices each comprise: areflective element, an emissive layer, a phase plate, and a polarizer inthis order, and further comprise a polarization separator between theemissive layer and the phase plate, which separates light emitted by theemissive layer into two kinds of polarized light components, wherein thepolarization separator has a reflectance of the wavelength range of ablue light higher than a reflectance of a light having a wavelength of555 nm, and wherein the emissive layer of at least one of the red andgreen organic light-emitting devices comprises a phosphorescentmaterial.
 25. The display device according to claim 21, wherein saidpolarization separator comprises a cholesteric liquid crystal layer, andsaid phase plate comprises a quarter wave plate, and said polarizationseparator, said phase plate, and a polarizer plate are provided from theside of said emissive layer in this order.
 26. The display deviceaccording to claim 25, wherein said polarization separator comprises acholesteric liquid crystal layer substantially comprising one kind of ahelical pitch, and the central wavelength of a selective reflection isbetween 400 nm to 490 nm.
 27. The display device according to claim 21,wherein an antireflection member for at least decreasing the reflectionof the light having the main wavelength range reflected by saidpolarization separator is provided on a non-emmissive area of the pixelcomposed of said light-emitting device.
 28. The display device accordingto claim 25, wherein said polarization separator comprises a pluralityof cholesteric liquid crystal layers each having a different helicalpitch, and the central wavelength providing a maximum selectivereflection is between 400 nm to 490 nm.
 29. The display device accordingto claim 28, wherein said plurality of cholesteric liquid crystal layersmaking up said polarization separator are stacked.
 30. The displaydevice according to claim 25, wherein said polarization separatorcomprises a cholesteric liquid crystal layer whose helical pitch iscontinuously changed.
 31. The display device according to claim 21,wherein said polarization separator is a linear polarization separator,which reflects a linearly polarized light having a prescribed wavelengthrange, and transmits lights other than said linearly polarized lighthaving a prescribed wavelength range; said phase plate comprises aquarter wave plate, and said phase plate, said polarization separator,and a polarizer plate are provided from the side of said emissive layerin this order.
 32. The display device according to claim 21, whereinsaid light-emitting device comprises an organic light-emitting diodehaving an electrode also serving as the reflective element, an emissivelayer comprising organic thin films, and an optical transparentelectrode stacked with each other.
 33. The display device according toclaim 21, wherein a space sealed with a gas is provided between aprotective layer and said polarization separator and the distancebetween said space and said emissive layer is quarter the wavelength ofthe light emitted from the emissive layer or less.
 34. The displaydevice according to claim 21, wherein said emissive layer substantiallymaintains the state of the polarization of the light transmittedthere-through, and said reflective element at least reflects acircularly polarized light impinging in the vertical direction mainly asa circularly polarized light having a reverse helicity direction.